A well known problem in the art of photogate CMOS and CCD image sensors is that of reduced short wavelength optical response. More specifically, one photogate circuit disclosed in U.S. Pat. No. 5,576,763, issued to Ackland, et al. on Nov. 19, 1996 achieves very low read noise. As disclosed, the main photosensitive region of the photogate circuit is the depletion region under the photogate and the region that is within the minority carrier diffusion length of the edge of that depletion region. The photogate is made large in area so as to maximize the number of collected electrons. However, the photogate sensor circuit suffers from poor quantum efficiency for short wavelength light due to the absorption characteristics of the polysilicon gate material employed. This problem is aggravated by the use of silicided gates which are used in many current CMOS fabrication processes. Photogate CCD image sensors, e.g. frame transfer CCD's, also suffer from this problem.
Photodiode-based image sensors have a better short wavelength optical response properties. However, CMOS photodiode-based image sensors suffer from a problem with reset noise. Specifically, it is impractical to save the actual reset value for the sensor, which is subtracted from the sensor output to eliminate the effect of reset noise from the sensor output. Therefore, a simulated reset value for a sensing cycle is developed using the output of the sensor after it is reset for the next sensing cycle. However, such simulated reset values may not accurately represent the actual reset value for the sensing cycle in which they are used, resulting in a poorer image quality.